Albert Barnes Commentary Joshua 22:10

"And when they came unto the region about the Jordan, that is in the land of Canaan, the children of Reuben and the children of Gad and the half-tribe of Manasseh built there an altar by the Jordan, a great altar to look upon." — Joshua 22:10 (ASV)

The two and a half tribes erected this altar to maintain their claim to have the same interest in God's sanctuary as the other tribes, which was established on the west side of the Jordan; and to prevent any assertion that the Jordan itself was a natural barrier excluding them from the sanctuary, they built it on the west or Canaanite bank of the Jordan, not on the east.

The word translated as “borders” is noteworthy; it means circuits, arrondissements.